In monitoring apparatus described in U.S. Pat. No. 4,152,655 each new pulse in the pulse sequence being monitored (input signal sequence) starts the countdown of a counter from a preset value at a frequency substantially higher than the input signal frequency. The count on the counter at receipt of the next input pulse is a measure of the actual input signal frequency and therefore can be used to determine the deviation of the input signal frequency from a desired input signal frequency. This type of apparatus requires a great deal of equipment. Because of this, its usefulness in monitoring sensors in a motor vehicle is quite limited. This type of apparatus has the further disadvantage that, when a microprocessor is used, a great many storage locations are required.